1. Field of the Invention
The present invention relates to a crystallized glass substrate for achieving a higher recording density in future information magnetic recording medium and which has properties required for future information magnetic recording medium such as lower specific gravity, higher Young's modulus, superior fracture toughness, remarkably smooth surface roughness after processing, excellent head sliding properties, and excellent impact resistance.
In this connection, the term “information recording medium” referred to in the present application means an information magnetic recording medium which can be used in hard disks for various electronic devices.
2. Related Art
In recent years, large data files such as movie and audio files have been used in personal computers and various electronic devices; therefore, information recording devices with larger capacity have been required. As a result, there has been a demand for higher recording density of information recording media year by year.
To contribute to satisfying this demand, a perpendicular magnetic recording system has been employed and mass production thereof has been advanced. In the perpendicular magnetic recording system, heat resistance and surface smoothness of substrates are required to a level higher than that of current substrates. It is also more important nowadays that specific gravity be lowered to alleviate load placed on spindle motors, mechanical strength be enhanced to prevent disks from being damaged, and fracture toughness be enhanced to improve impact resistance with heads when dropped.
The material used for substrates of information recording media is exemplified by Al alloy, glass, and crystallized glass. Glass and crystallized glass are superior to Al alloys in view of higher Vickers hardness, higher surface smoothness, and the like, and thus have often been employed in applications where dynamic use is envisaged.
However, the crystallized glass in current use is provided with higher mechanical strength by virtue of crystals precipitated in glass phase, on the other hand, the crystallized glass exhibits a difference in processing between precipitated crystals and glass phases or a difference in etching rate. Therefore, the crystallized glasses currently being employed do not sufficiently satisfy requirements of surface roughness of Ra<2 angstroms, which is required of next generation substrates.
Furthermore, as the glass is brittle, defects originating at microcracks in the surface of substrates are likely to occur. Particularly, with the substrates for information recording medium used for the next generation of hard disks, resistance to crack propagation originating from microcracks in the surface of substrates, i.e., fracture toughness, has become an especially important evaluation point since the rotational speed of magnetic disks tends to be higher along with higher recording density. Therefore, the substrates for information recording medium are required to have higher fracture toughness. However, the glass substrates currently being employed do not easily satisfy such requirements, thus it is necessary to perform a step of chemical strengthening of the substrates after polishing.
Furthermore, when a material of glass or crystallized glass is used, a direct press process is employed to directly press molten glass for the purpose of producing a disk-shaped substrate having a thickness of 1 mm or less at lower cost.
In the direct press process, heretofore, arsenic or antimony component has been contained in the raw material as a clarifying agent to remove bubbles from the molten glass when melting the glass. However, it has been required in recent years to reduce the content of arsenic or antimony component or not to use them due to the possibility of adversely affecting human bodies and the environment. Nonetheless, when the arsenic or antimony component is substantially not used in the raw material and other clarifying components are used, there arises a problem in that bubbles in glass base material are generated or reboil during direct press occurs.
Patent Document 1 discloses a substrate for information recording medium consisting of a crystallized glass having a gahnite crystal phase. The substrate has a high fracture toughness, but surface roughness after polishing does not satisfy the level required for the substrate for information recording medium of the next generation. It is also impossible for the substrate for information recording medium of Patent Document 1 to satisfy the production cost required by the market due to lower polishing rate because of excessively high surface roughness and lower productivity because of requiring longer time for the polishing process.
It is also very difficult for the crystallized glass to control degree of crystallinity and grain size of precipitated crystal because of the inclination to rapidly generate crystal precipitation when crystallizing the raw glass through heat treatment. Details thereof will be explained later.
Patent Document 2 discloses a glass ceramic which contains a spinel type compound as a crystal phase. However, the melting temperature of the glass in Examples of Patent Document 2 is as high as from 1500 degree C. to 1650 degree C. and the highest crystallizing temperature is in a range of from 950 degree C. to 1000 degree C., therefore, mass productivity thereof is poor. In addition, surface roughness Ra of the resulting glass ceramic is 5.3 angstroms, which does not satisfy the desired surface property.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. H07-300340    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H09-077531